In recent years, in the field of optical communications, as transmission capacity increases, the intensity (power) of light which is propagated in optical fibers increases. In addition, in optical fiber lasers, as the laser output of the optical fiber lasers increases, high-power light in a range from several hundred W to several thousand W is propagated in the optical fibers.
In optical fibers in which the high-power light is propagated, there is a possibility that a fiber fuse occurs due to overheating caused by dust and the like attached to an end surface thereof or overheating caused by local-bending of the optical fiber, resulting in damage not only the optical fibers but also devices or apparatuses connected to the optical fibers (for example, refer to Non-Patent Documents 1 and 2).
FIGS. 1 and 2 respectively show a side view and a cross-sectional view illustrating a single mode optical fiber (SMF) through which the fiber fuse passes. In the drawings, the reference numeral 10 represents an optical fiber, the reference numeral 11 represents a core, and the reference numeral 12 represents a cladding. As shown in the drawings, in the optical fiber 10 through which the fiber fuse passes, voids 1 periodically occur in the center core 11. Since the voids prevent the propagation of light through the optical fiber, the passage of the fiber fuse is a fatal obstacle to the communication system, the optical fiber laser, and the like. Once the fiber fuse occurs, it will continue to pass through the optical fiber and the waveguide structure of the optical fiber will be damaged unless the intensity of the light propagating in the optical fiber drops below a threshold value. The threshold of the optical intensity varies depending on the structure of the optical fiber and the like. In the present specification, the threshold value of the optical intensity for terminating the fiber fuse is referred to as the “fiber fuse threshold value”.
As techniques for terminating the fiber fuse midway along the optical fiber in order to protect optical transmission lines or apparatuses, the following techniques are known.
Patent Document 1 describes a technique of terminating the fiber fuse in which power density in the core is reduced by partially expanding a mode field diameter (MFD) of a part of a single mode optical fiber.
Patent Document 2 describes an optical fiber transmission line in which a graded index (GI) optical fiber is inserted midway on the optical fiber transmission line to create an enlarged-core portion, thereby terminating the fiber fuse phenomenon.
Patent Document 3 describes a technique of terminating the fiber fuse phenomenon by providing an optical attenuator of a photonic crystal fiber type midway on the transmission line.
Non-Patent Document 3 describes that the fiber fuse can be terminated by etching a cladding of an optical fiber to thin the outer diameter of the optical fiber to approximately twice the MFD. For example, in the case where the MFD is 9.5 μm, when the outer diameter is 10.5 to 33 μm, the fiber fuse can be terminated. In addition, Non-Patent Document 3 describes that the outer diameter of the etched portion of the optical fiber required for terminating the fiber fuse has little effect on the emission strength of the laser.
Non-Patent Document 4 examines the characteristics, with respect to fiber fuses, of a “microstructured fiber” which is provided with a center portion surrounded with 30 holes (having diameters of approximately 1 μm, and a center-to-center distance of approximately 2 μm) and allows single-mode propagation with MFD of approximately 2 μm at a wavelength of 1.06 μm. According to Non-Patent Document 4, the fiber fuse threshold value of the “microstructured fiber” is more than 10 times that of a conventional SMF having approximately the same MFD.
As a fusion-splice technique of a hole-assisted optical fiber (HAF) which includes at its center a core with a refractive index higher than a cladding and holes in the cladding, the following technique has been known.
Non-Patent Document 5 describes a technique that intermittent discharge or sweep discharge is performed on an optical fiber in which holes are disposed around a core of a general SMF to collapse the holes in a tapered shape, so that the optical fiber is fusion-spliced to the SMF with an average splicing loss of 0.05 dB.